Use is made of the variation in the conductivity of such metal oxides both as a function of temperature and as a function of the presence of certain gases for the purpose of detecting trace or impurity gases in an atmosphere being monitored.
One of the factors influencing this variation and relating to the presence of a gas is chemisorption i.e. the transfer of electrons between the gas and the oxide. Thus the absorbed gas either takes electrons from the semiconductive oxide, or else it gives them up to it. Depending on the P or N type of the semiconductor and the donor or acceptor character of the gas, the conductance increases (P & acceptor or N & donor) or decreases (N & acceptor or P & donor). It will be understood that sensor sensitivity depends on there being as many occasions as possible for exchange to take place between the gas and the semiconductor. For a given quantity of deposited oxide, it is necessary to have as large an exchange area as possible, and thus to look for ways of making the layer as uncompact as possible.
Known methods of manufacturing such sensors are based on vacuum depositing a semiconductive metal oxide (e.g. tin dioxide: SnO.sub.2). In outline the procedure is as follows. A vacuum is established down to about 10.sup.-4 Pa in a chamber containing an insulating substrate and a crucible raised to a temperature lying in the range 1100.degree. C. to 1300.degree. C., and a flow of oxygen at about 10 Pa to 20 Pa is injected. SnO.sub.2 is deposited on the substrate situated in the chamber at a few centimeters from the crucible, with deposit being in a layer having the well-known column structure which, while not compact, does not offer sufficient sites for chemisorption activity for a sensor of this type to be very great sensitivity.
If the oxygen pressure is increased at the moment of evaporation to as much as 130 Pa or more, the structure of the deposited layer changes and becomes spongy, which is more favorable for obtaining acceptable sensitivity. However, such high oxygen pressure causes the refractory metal of the crucible to oxidize, with the resulting oxides being entrained towards the substrate where they pollute the deposited spongy layer, thereby destroying the sensitivity of the sensor.